Method of forming an electrically conductive cellulose composite

ABSTRACT

An electrically conductive cellulose composite includes a cellulose matrix and an electrically conductive carbonaceous material incorporated into the cellulose matrix. The electrical conductivity of the cellulose composite is at least 10 μS/cm at 25° C. The composite can be made by incorporating the electrically conductive carbonaceous material into a culture medium with a cellulose-producing organism, such as  Gluconoacetobacter hansenii . The composites can be used to form electrodes, such as for use in membrane electrode assemblies for fuel cells.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

The United States Government has certain rights to this inventionpursuant to Contract No. DE-AC05-000822725 between the United StatesDepartment of Energy and UT-Battelle, LLC.

FIELD OF THE INVENTION

This invention relates generally to cellulose composites, and moreparticularly to electrically conductive cellulose composites, andmethods of making such composites.

BACKGROUND OF THE INVENTION

Cellulose is the most abundant biologically synthesized polymer onearth. It is composed of monomers of the sugar glucose that are joinedinto long chains by covalent beta (1,4) glycosidic linkages that areformed between the C₁ aldehyde and C₄ hydroxyl groups of the glucosemolecule. This chemical structure imparts to cellulose its crystalline,fibrous physical structure. Various forms of cellulose are majorindustrial agricultural products, particularly cotton and wood.Cellulose is the major component of many products such as paper,textiles, cardboard, construction materials, and many other products.Most cellulose is obtained from the familiar multi-cellularphotosynthetic terrestrial plants. Cellulose is also produced in theoceans by unicellular plankton or algae using the same type of carbondioxide fixation found in the photosynthesis of land plants.

Certain bacteria can assemble cellulose via non-photosynthetic pathways,requiring glucose, sugar, glycerol, or other organic substrates forconversion into pure cellulose. One such bacterium is Acetobacterxylinum, now taxonomically classified as Gluconacetobacter xylinus. Asingle Acetobacter xylinum cell can convert up to 108 glucose moleculesper hour into cellulose. The Acetobacter cells produce sub-microscopiccellulose fibrils which gather in an entangled mesh to produce agelatinous membrane known as a pellicle. The microbial cellulose soformed benefits from the absence of lignin or hemicelluloses, and iscompletely biodegradable and recyclable. Microbial cellulose alsoprovides high strength, consistent dimensional stability, high tensilestrength, light weight and excellent durability. It is also extremelyabsorbent in the hydrated state.

Another advantage of microbial cellulose is the potential for directmembrane assembly during biosynthesis. The medium can be suspended in amold or desired shape to directly form useful products. Extremely thin,sub-micron, optically clear membranes can be assembled. Intermediatesteps of paper formation from pulp are unnecessary, and textile assemblyfrom yarn is unnecessary. Cellulose orientation during synthesis ispossible for dynamic fiber forming capabilities, and uniaxiallystrengthened membranes. Crystallization can be delayed by theintroduction of dyes into the culture medium, and the physicalproperties of the cellulose such as molecular weight and crystallinitycan be controlled. Also, from this cellulose the direct synthesis ofcellulose derivatives such as cellulose acetate, carboxymethylcellulose,methyl cellulose and other derivatives is possible. It is also possibleto control the cellulose crystalline allomorph (cellulose I or celluloseII). Brown, Jr., et al., U.S. Pat. No. 4,954,439 disclose acellulose-producing microorganism which is capable, during fermentationin an aqueous nutrient medium containing assimilable sources of carbon,nitrogen and inorganic substances, of reversal of direction of thecellulose ribbon extrusion such that a cellulose ribbon-bundle having awidth of at least two cellulose ribbons is produced.

Microbial cellulose is not intrinsically electrically conductive.Various efforts have been made to impart electrical conductivity to suchcellulose, including thermal transformation, deposition of metallic orother conductive particles, and infusion of dyes Yoshino et al. inSynthetic Metals 41-43, 1593-1596 (1991) describe the pyrolyzation ofbacterial cellulose at temperatures from 1000 to 3000° C. to transformit to a graphite material. Evans, et al, Pub. No. U.S. 2003/0113610discloses a method for the deposition of metals onto bacterialcellulose, where the bacterial cellulose matrix is placed in a solutionof a metal salt such that the metal salt is reduced to the metallic formprecipitates in or on the cellulose matrix.

SUMMARY OF THE INVENTION

An electrically conductive cellulose composite comprises a cellulosematrix, and an electrically conductive carbonaceous materialincorporated in the cellulose matrix. The concentration of carbonaceousmaterial is sufficient to provide an electrical conductivity for thecomposite of at least 10 μS/cm at 25° C. Catalytic metals such aspalladium can be subsequently added to the composite by catalyticdeposition. The electrical conductivity and structural properties ofcarbon-cellulose composites according to the invention enable its use ina variety of applications including as a fuel cell electrode in apolyelectrolyte membrane (PEM) fuel cell and as an electrode in asensor.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments which are presentlypreferred, it being understood, however, that the invention is notlimited to the precise arrangements and instrumentality shown, wherein:

FIG. 1 is a scanned scanning electron microscope (SEM) image of acellulose composite according to an embodiment of the invention havingembedded graphite particles therein.

FIG. 2 is a schematic diagram depicting a membrane electrode assembly(MEA) according to the invention.

FIG. 3 is a graph depicting current production from a proton exchangemembrane (PEM) fuel cell formed from a MEA based on electricallyconductive cellulose composites according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

A cellulose-producing organism is cultured under appropriate conditionsto cause the cellulose-producing organism to produce an electricallyconductive cellulose composite, or a composite material that uponappropriate heating can be rendered electrically conductive. Acarbonaceous material, preferably being an electrically conductivematerial such as graphite particles, is supplied to the medium, suchthat the organism will incorporate the electrically conductedcarbonaceous material throughout the cellulose matrix of the pellicule.A cleaning procedure can be used to remove the organism, leaving theelectrically conductive carbonaceous material entrapped in thecomposite. The cellulose composite can be dried to a thin membrane, orother desired shape.

The cellulose composite generally includes 10 to 70% w/wcarbon/cellulose. The bulk electrical conductivity of the cellulosecomposite is at least 10 μS/cm at 25° C., such as at least 1 mS/cm, 10mS/cm, 100 mS/cm, or most preferably at least 1 S/cm at 25 C.

The incorporation of graphite or other carbonaceous materials into thecellulose renders the material electrically conductive. This electricalconductivity has been found to be preserved during subsequent depositionof a catalyst layer, such as palladium, on the surface of the composite.Neither bacterial cellulose without carbonaceous additives nor bacterialcellulose containing only catalysis such as palladium on its surface hasbeen found to be electrically conductive after drying to membranes.Significantly, catalyst comprising cellulose composites according to theinvention have been demonstrated to function as a combinationelectrode/current collector for a fuel cell without the necessity of theinsertion of platinum wires for attachment of the multimeter leads asrequired by earlier related work, as described below.

The catalyst layer can also be deposited by the organism. For example,palladium can be deposited in graphite-containing cellulose by additionof hexachloropalladate solution to a culture of live cellulose producingbacteria that has formed a cellulose pellicule incorporating graphiteparticles. In this embodiment, the palladium particles are nucleated byreduction of the hexachloropalladate by the cellulose reducing ends,which are aldehyde groups with an E° of about 0.450 V.

FIG. 1 is a scanned scanning electron microscope (SEM) image of acellulose composite according to an embodiment of the invention havingembedded graphite particles therein. The graphite particles 105 can beseen to be intermixed with the cellulose fibers 110. It is believed thatthe bacteria are delivered to extrude their cellulose microfibrilsaround the graphite particles, entrapping them in a netlike matrix ofcellulose. The graphite particles are in sufficiently close proximity toenable electron conduction between the graphite particles, as well asbetween the graphite and particles of palladium of size about 2-20 nmthat can be subsequently formed by catalytic reduction carried out bythe cellulose reducing ends as described in Evans, et al, Pub. No. U.S.2003/0113610. The invention thus provides cellulose composites having anessentially continuous electrically conductive network throughout.Further evidence of the continuous electrically conductive network canbe found in the electrical conductivity data presented in the examplesbelow.

The cellulose-producing organism can be any suitable such organism,either now in existence or a new, genetically modified organism. Thebacteria should be capable of producing sufficient quantities ofcellulose, and must also be capable of incorporating the carbonaceousmaterial into the cellulose matrix. The cellulose-producingmicroorganism can in one aspect be selected from the genera Acetobacterand Gluconacetobacter. Species of Acetobacter include Acetobacter aceti,Acetobacter hansenii, Acetobacter xylinum or Acetobacter pasteurianus.Currently preferred organisms include Gluconoacetobacter hansenii (ATCC10821) and Gluconoacetobacter xylinus. The taxonomic identification ofthese bacteria has been described by Yamada et al., (1997), “Thephylogeny of acetic acid bacteria based on the partial sequences of 16Sribosomal RNA: the elevation of the subgenus Gluconoacetobacter to thegeneric level”, Biosci. Biotechnol. Biochem. 61(8): 1244-1251.

The carbonaceous material can be any material which is or can be made tobe electrically conductive and can be incorporated by the organism intothe cellulose matrix. The carbonaceous material is generally added in aweight % range of 1 to 5% w/v for addition to the growth medium. Thecarbonaceous material must have suitable size, composition, and shape inorder that the organism can effectively incorporate the material intothe cellulose matrix. Graphite powder is one suitable such material.Other suitable carbonaceous materials include activated charcoal,activated carbon, carbon nanotubes, carbon nanofibers, activated carbonfibers, graphite fibers, graphite nanofibers, and carbon black.

The carbonaceous material provided to the organism is preferablyelectrically conductive. Graphite, powders with particle size of 2-50microns, for example, have an electrical conductivity at roomtemperature that has been reported to lie in the range of 10-1000 S cm⁻¹for compacted samples (N. Deprez & D. S. Mc Lachlan, J. Phys. D: Appl.Phys. 21, 101-107, 1988). It is also possible to treat the carbonaceousmaterial so as to make the carbonaceous material electrically conductiveor more electrically conductive. Such treatment can occur before orafter incorporation of the carbonaceous material into the cellulosematrix by the organism. For example, regarding treatment afterincorporation, bacterial cellulose containing incorporated carbonaceousmaterial can be pyrolyzed to a graphite or an electrical conductiveamorphous material by heating to temperatures such as 200-1200° C. in anon-oxidizing environment. Similarly, bacterial cellulose into whichcarbonaceous material is incorporated, followed by deposition of acatalyst such as palladium, nickel, platinum, ruthenium, gold, or silveron the same cellulose matrix, can be pyrolyzed to a graphite or anelectrically conductive amorphous material by heating to about 200-1200°C. in a non-oxidizing environment.

The culture medium for the cellulose-producing organism can be optimizedfor the particular organism that is being cultured. The culture mediumshould supply the organism with sufficient nutrients such that thecellulose synthesis can take place effectively and efficiently.Synthetic and rich media for the cultivation of Gluconoacetobacterhansenii are shown in Table 1. Other media can be used. The growth ofthe bacteria can be carried out as described above using Schramm-Hestrinmedium with 2% mannitol or fructose substituted for the 2% glucose,using Schramm-Hestrin medium with soy peptone substituted for thebactopeptone, or using a synthetic medium containing 2% glucose,fructose, or mannitol supplemented with niacinamide, thiamine, andcalcium pantothenate.

TABLE 1 Two media formulations used by the inventors for production ofbacterial cellulose. Schramm-Hestrin Modified Synthetic    2% glucose   2% glucose  0.5% yeast extract  0.1% ammonium chloride  0.5% peptone0.115% citric acid  0.27% disodium phosphate  0.33% sodium dihydrogenphosphate 0.115% citric acid  0.01% potassium chloride 0.025% magnesiumsulfate 100 mg 1⁻¹ niacinamide 100 mg 1⁻¹ calcium pantothenate 100 mg1⁻¹ thiamine pH 6.2 pH 6.2The synthetic media formulation was modified from that described in“Biogenesis of Bacterial Cellulose”, R. E. Cannon and S. M. Anderson,Microbiology 17(6): 435-447 (1991).Schramm-Hestrin media was modified from that described in S. Schramm andM. Hestrin, Biochem. J. 57: 345-352 (1954), “Synthesis of Cellulose byAcetobacter xylinum. 2. Preparation of freeze-dried cells capable ofpolymerizing glucose to cellulose.”

The vitamins niacinamide, calcium pantothenate, and thiamine areprepared as a 1000× stock solution, filter-sterilized, and added to theautoclaved media after it is cooled to room temperature. Withoutadjustment, the pH is about 6.4. The bacteria are cultured undertemperature and pH conditions which are suitable to facilitate thecellulose synthesis and incorporation of the carbonaceous material. Asthe bacteria synthesize the cellulose on the top of the liquid media,the graphite or other carbon particles are added after an initialcellulose layer has been synthesized during the incorporation underthese static culture conditions due to the effects of gravity on thegraphite particles causing them to fall to the bottom of the culturedish.

As noted above, electrically conductive cellulose compositions accordingto the invention can be used to form a number of different products.These products include membranes of various thickness and dimensions.The compositions can be combined with other materials to alter theproperties of the final product. For example, a layer of metal can bedisposed on and in contact with the cellulose composites of theinvention. Such metal deposition techniques are known in the art, andcan include those shown in Pub. No. US 2003/013610, the disclosure whichis incorporated by reference. The incorporated metal can be suitable ametal, such as the catalyst, Pd, Pt, Ru, Au or Ni. Other materials canbe incorporated into or on the cellulose composite.

For example, the catalyst-containing cellulose composite can be used toform membrane electrode assemblies (MEA) for use in fuel cells. There isshown in FIG. 2 a MEA 10 having electrodes (anode, cathode) 14,18. Theelectrodes 14,18 are formed from electrically conductive cellulosecompositions of the invention, with catalyst particles (now shown)formed within the cellulose matrix, such as palladium particles.Although not shown in FIG. 2, the catalyst particles can be a discretelayer coating one side of the electrodes 14, 18, as is observed withvapor deposition or with electroplating. A proton exchange membrane(PEM) 22 is disposed between the catalyst comprising electrodes 14,18.The PEM 22 can be any suitable material, and can comprise cellulose. MEA10 also includes anode lead 15 for connection to anode 14 and cathodelead 17 for connection to cathode 18. Electrically conductive cellulosemembranes according to the invention can also be used in the assembly ofmembrane electrode assemblies for use in electronic devices, fuel cellsand biosensors.

EXAMPLES

The present invention is further illustrated by the following specificExamples, which should not be construed as limiting the scope or contentof the invention in any way.

The formation of electrically conductive cellulose composites accordingto the invention is illustrated by the following example.Gluconoacetobacter hansenii (ATCC 10821) is cultivated inSchramm-Hestrin medium (2% glucose, 0.5% bactopeptone, 0.5% yeastextract, 0.27% disodium monohydrogen phosphate, 0.115% citric acid, pH6.2) under static conditions in shallow culture dishes at 23-28° C.Typically, 10 ml of inoculated culture medium are placed in a 6-cmculture dish or 20 ml in a 10 cm culture dish. After the bacteria havestarted to form a clear, gel-like cellulose pellicule, usually about 3days, 0.5 g sterilized graphite (Alfa Aesar, Ward Hill, Mass.,electronic grade, 2-15 microns in size) is suspended as a slurry inculture medium, then added to the bacterial culture on top of thecellulose layer. Incubation is continued until the graphite particleshave become entrapped in the growing cellulose, about 5-10 days.

The cellulose pellicule is then harvested by removal from the culturedish to a heat-resistant container for cleaning to remove the organismand media components. The cellulose is first heated in distilled waterto 90-100° C. for 1-2 h, then washed with 3 changes of distilled water.It is then incubated in 1% sodium hydroxide solution for 1-3 days. Thesodium hydroxide is then neutralized by the addition of 1.5 volumes of0.5 M sodium acetate buffer, pH 4.5. Acetate is removed by soaking thecellulose in distilled water. The cellulose is then stored in 20%ethanol or isopropanol. Other cleaning routines can be employed, such asheating the harvested cellulose in distilled water in an autoclave, andutilization of other alkali solutions such as Trizma base and ammoniumhydroxide, and other acids for neutralization such as hydrochloric acidand phosphoric acid. When activated charcoal (medium grade) issubstituted for the graphite in the procedure described above, it isincorporated into the cellulose pellicule by the bacteria in a similarmanner to that observed for the graphite particles.

The cellulose pellicule containing graphite incorporated in vivo isincubated in a solution of hexachloropalladate for preparation of acellulose/graphite/palladium membrane according to the invention. Theresistance of the oven-dried pellicules was measured using a Wiley 200Multimeter equipped with standard copper leads at each electrode. Asexpected, the electrical resistance was lower (electrical conductivityhigher) for the cellulose containing graphite than that of the cellulosecontaining activated charcoal. Infinity (oo) represents an electricalresistivity too high to measure and μS=microSiemens.

TABLE 2 Electrical Conductivity of Graphite-Cellulose MembranesElectrode Resistivity Conductivity Sample Measured Separation (cm)[Mohm] [μS/cm] Bacterial Cellulose no additives 10 ∞ 0 ActivatedCharcoal Bacterial Cellulose 5 0.10  10 Activated Charcoal BacterialCellulose 2 ∞ 0 Palladium (one palladization incubation) ActivatedCharcoal Bacterial Cellulose 2 ∞ 0 2Palladium (two palladizationincubations) Bacterial Cellulose Palladium 6 ∞ 0 Bacterial CellulosePalladium 6 ∞ 0 Graphite Bacterial Cellulose 10 0.0030 33.3 GraphiteBacterial Cellulose 10 0.0024 41.6 Graphite Cellulose Palladium 6 0.0011151.5

A fuel cell was assembled using electrodes comprising Pd coatedcellulose-graphite composite according to the invention. The protonconducting membrane layer was a bacterial graphite/cellulose pelliculethat had been soaked in a 1 M potassium chloride solution. Thepalladium-graphite electrode layers were used as the electrode, currentcollector and catalyst. The layers were assembled by sequential drawingon a standard dryer with heating to 60° C. for 10 minutes. The testedfuel cell assembly had final dimensions of 2 cm×2 cm×1 mm. The fuel cellassembly was tested by placing it between rubber O-ring seals connectinga glass container containing 3 ml of 10% acetic acid in water and 180 mgof iron powder and an open glass tube. The acetic acid and iron filingsgenerate H₂ by the acid displacement reaction. The open tube allows freediffusion of O₂ from the atmosphere to the anodic side of the fuel cell.This testing apparatus was described previously in Evans, et al, Pub.No. U.S. 2003/0113610. The current output for a singlepalladium-graphite cellulose fuel cell formed as described above isshown in FIG. 3.

While the preferred embodiments of the invention have been illustratedand described, it will be clear that the invention is not so limited.Numerous modifications, changes, variations, substitutions andequivalents will occur to those skilled in the art without departingfrom the spirit and scope of the present invention as described in theclaims.

1-12. (canceled)
 13. A method of forming electrically conductivecellulose composites, comprising the steps of: providing acellulose-producing bacterium in cultivated conditions including asuitable culture medium, and adding carbonaceous material to saidculture medium at a point at which the bacterium has initiated celluloseproduction to form a cellulose matrix, wherein said bacteriumincorporates said carbonaceous material in said cellulose matrixproduced to provide an electrically conductive cellulose composite,wherein an electrical conductivity of said composite provides anelectrical conductivity of at least 10 μS/cm at 25° C.
 14. The method ofclaim 13, wherein said carbonaceous material is electrically conductivematerial.
 15. The method of claim 13, wherein said carbonaceous materialis non-electrically conductive material, further comprising the step ofheating at a temperature between 200 and 1200 C to form saidelectrically conductive cellulose composite.
 16. The method of claim 13,wherein a weight % range for said carbonaceous material is 1-5% w/v foraddition of said culture medium.
 17. The method of claim 16, whereinsaid electrically conductive cellulose resultant composite contains 10to 70% w/w carbon/cellulose.
 18. The method of claim 13, wherein saidbacterium is of the genus Gluconoacetobacter or Acetobacter.
 19. Themethod of claim 13, further comprising the step of adding a metalprecursor solution to said culture medium, wherein metal particles areformed within said cellulose matrix.
 20. The method of claim 13, whereinsaid metal precursor solution comprises a Pd, Pt, Ru, Au, or Niprecursor solution.